But - unless I've misunderstood - (1) depend upon the PCs gaining information about their possibility in the course of play (eg talking to NPCs), and therefore (2) depend upon the GM deciding what is part of the fiction, what the various NPCs whom the PCs are in contact with know and might communicate, etc.
If the party have done their diligence and found 4 or 5 different possibilities for adventure and then decide as a party to ignore them all and instead go north to take over the barbarian tribes then - unless I in fact do want to railroad them for some reason - I'm DMing an attempted takeover of the barbarian tribes. And this might be all complete speculation on the players'/PCs' part - "Hey, lets go see if there's any barbarian tribes we can take over - we can make 'em into our own private army!" - without any actual knowledge of whether such tribes exist or if they do what they might have going for them.
But if just one character wants to do this while the rest would rather go and see what's up with Dumont tower then one of several things must happen:
1. Party goes to Dumont, single PC leaves party and goes north (and retires from ongoing play either temporarily or permanently, meanwhile I run Dumont); or
2. A deal is struck: you help us with Dumont and we'll help you with the tribes (in whichever order, I run both); or
3. The one PC manages to talk the party out of doing Dumont at all and convinces them to go north (thus I run just the tribes adventure); or
4. The party splits - some go to Dumont, some go north (I run one first [probably Dumont as it's closer*] while the other gets put on hold, then I run the other)
* - for game-world timing reasons I'd always want to run the closer one first; it's entirely possible the Dumont group finish their adventure (which is a mere 25 miles away) before the northern group even get to the barbarians who are hundreds of miles away. I'd meta-explain this to the players - never yet had any trouble getting buy-in on something like this - as there's always a chance, however tiny, that something done at Dumont may somehow affect those going north.
Lan-"and yes, barbarians are a race not a class"-efan